Magnetic resonance tomography (MR) requires the ECG signals of the patient to be detected in order to synchronize the triggering of the MR measurement sequences to the heartbeat of the patient. Similarly, the information relating to the current heart phase can be obtained by means of the ECG signal detected during an MR examination. If the ECG signals and the triggering and/or activation of the measurement sequence are not synchronized, there is a risk of the MR images containing movement artifacts.
In practice however, the detection of ECG signals is associated with difficulties, since the electrical and magnetic fields acting during the MR sequences couple into the ECG electronics system in a significantly interfering fashion, as a result of which the reliable determination of the heart phase is adversely affected. Besides these unwanted couplings into the ECG electronics system, the so-called magnetohydrodynamic effect occurs in the case of higher magnetic flux densities, which results in the T-wave of the heartbeat being overshot. The different phases of the heart cycle are designated in the field of electrocardiography with letters, by the sequence P-Q-R-S-T for instance. In this process, the R-wave shows the greatest amplitude, it constitutes the reference point of the triggering and must therefore be determined in a reliable fashion.
A method for the ECG triggering of a measurement sequence of a magnetic resonance device is known from U.S. Pat. No. 6,070,097, but the ECG signals of a patient are detected in this method via a single channel.
WO 99/04688 has proposed the detection of ECG signals via two channels, a vector display in a coordinate system is derived herefrom. The R-wave of the heart cycle is to be inferred from this display. It is however doubtful whether this method features adequate reliability, since this vector projection is dependent on many influences, it changes for instance if the patient holds his/her breath.
In the non pre-published application DE 10 2005 027 438 A1, a further improvement has been proposed in that the triggering of the measurement sequence is carried out if the ECG signal of at least one channel which is subjected to digital signal processing exceeds a threshold value and the derived value of the ECG signal or the sum of the derived values of the ECG signal lies within a defined interval in the case of both channels. The measurement sequence of the magnetic resonance device is then only triggered when a number of conditions are fulfilled at the same time.